The present invention relates to conveyors and in particular to a gravity conveyor having a velocity and back pressure control for items being conveyed.
Gravity conveyors comprise non-driven rollers or skate wheels that utilize a difference in elevation from the inlet to the discharge as a conveying force. Two problems exist which limit the use of gravity conveyors. A first problem is that heavy articles may travel at an excessively high velocity down the gravity conveyors. Heavy articles traveling at high speeds can cause damage to other downstream articles or be a hazard to people.
A second problem concerns a back pressure. That is, if articles are blocked from exiting a gravity conveyor and more articles are introduced into the inlet, back pressure acting against the downstream article(s) grows as the gravity conveyors fills. Thus the articles near the exit of the gravity conveyor experience high pressure from the upstream articles.
In order to alleviate the first problem, it is necessary to reduce the speed of the heavy articles being conveyed. One conventional way of achieving this is to design at least some of the gravity rollers as so-called centrifugal brake rollers which include therein a braking mechanism that slows the roller (and thus the articles) in response to increased centrifugal force. Since the magnitude of centrifugal force is a function of the rotary speed of the roller, a certain speed control is established. However, centrifugal brake rollers are not suitable in systems which convey both heavy and light articles. That is, if the centrifugal brake rollers are sized to provide braking force for heavy articles, then the light articles may become hung up thereon due to the high friction. On the other hand, if the brake rollers are sized to enable light-weight articles to easily pass thereover, then the braking influence on heavy articles will be nominal. Also, a centrifugal braking roller does not alleviate the above-described back pressure problems, because it only functions to brake moving articles; stationary articles will not be subjected to an appreciable braking force.
A known technique for relieving back pressure involves sensing the weight of the articles. For example, in FIG. 8 there is depicted a gravity conveyor comprised of freely rotatable gravity rollers 10. The uppermost portions of the gravity rollers define a profile plane P'. A fluid actuated brake pad 12 is positioned to engage one or more of the gravity rollers. The brake pad is actuated by a sensor roller 14 that is mounted on a trip bar 15. The sensor roller 14 lies in the conveying path at an elevation above the profile plane P'. The article A, due to its weight, will depress the sensor roller and actuate a sensor 16 which is electrically connected to the brake pad 12 to cause the latter to be extended against an upstream gravity roller to decelerate an upstream article or hold it in place. This technique is relatively expensive and has limited utility when used in systems that convey articles which include light-weight articles that may become hung-up when they abut against the sensor roller, i.e. the light-weight articles may not be heavy enough to depress the sensor roller.
A weight-sensitive braking mechanism is also disclosed in De Good U.S. Pat. No. 3,321,057 wherein a sensing roller is spring biased upwardly to an elevation higher than the profile plane defined by the upstream gravity rollers. An article exceeding a predetermined weight will depress the sensing roller downwardly against a brake pad to apply a braking force to the sensing roller which is transmitted to the article. This, however, creates a problem similar to that disclosed earlier in that light-weight articles may become hung-up against the upstanding sensing roller. Also, this type of device does not alleviate back pressure.